Stable proton exchange membranes and membrane electrode assemblies

ABSTRACT

A proton exchange membrane and a membrane electrode assembly for an electrochemical cell such as a fuel cell are provided. A catalytically active component is disposed within the membrane electrode assembly. The catalytically active component comprises particles containing a metal oxide such as silica, metal or metalloid ions such as ions that include boron, and a catalyst. A process for increasing peroxide radical resistance in a membrane electrode is also provided that includes the introduction of the catalytically active component described into a membrane electrode assembly.

FIELD OF THE INVENTION

The present invention relates to proton exchange membranes, electrodes,and membrane electrode assemblies of an electrochemical cell, such as afuel cell, that include a catalytically active component capable ofdecomposing hydrogen peroxide, thereby providing a more stable protonexchange membrane and membrane electrode assembly. The invention alsorelates to a method for operating a membrane electrode assembly so as toincrease resistance to peroxide radical attack.

BACKGROUND

Electrochemical cells generally include an anode electrode and a cathodeelectrode separated by an electrolyte, where a proton exchange membrane(hereafter “PEM”) is used as the electrolyte. A metal catalyst andelectrolyte mixture is generally used to form the anode and cathodeelectrodes. A well-known use of electrochemical cells is in a stack fora fuel cell (a cell that converts fuel and oxidants to electricalenergy). In such a cell, a reactant or reducing fluid such as hydrogenor methanol is supplied to the anode, and an oxidant such as oxygen orair is supplied to the cathode. The reducing fluid electrochemicallyreacts at a surface of the anode to produce hydrogen ions and electrons.The electrons are conducted to an external load circuit and thenreturned to the cathode, while hydrogen ions transfer through theelectrolyte to the cathode, where they react with the oxidant andelectrons to produce water and release thermal energy.

Fuel cells are typically formed as stacks or assemblages of membraneelectrode assemblies (MEAs), which each include a PEM, an anodeelectrode and cathode electrode, and other optional components. Fuelcell MEAs typically also comprise a porous electrically conductive sheetmaterial that is in electrical contact with each of the electrodes andpermits diffusion of the reactants to the electrodes, and is know as agas diffusion layer, gas diffusion substrate or gas diffusion backing.When the electrocatalyst is coated on the PEM, the MEA is said toinclude a catalyst coated membrane (CCM). In other instances, where theelectrocatalyst is coated on the gas diffusion layer, the MEA is said toinclude gas diffusion electrode(s) (GDE). The functional components offuel cells are normally aligned in layers as follows: conductiveplate/gas diffusion backing/anode electrode/membrane/cathodeelectrode/gas diffusion backing/conductive plate.

Long term stability of the PEM is critically important for fuels cells.For example, the lifetime goal for stationary fuel cell applications is40,000 hours of operation. Typical membranes found in use throughout theart will degrade over time through decomposition and subsequentdissolution of the ion-exchange polymer in the membrane, therebycompromising membrane viability and performance. While not wishing to bebound by theory, it is believed that this degradation is a result, atleast in part, of the reaction of the ion-exchange polymer of themembrane and/or the electrode with hydrogen peroxide (H₂O₂) radicals,which are generated during fuel cell operation. Fluoropolymer membranesare generally considered more stable in fuel cell operations thanhydrocarbon membranes that do not contain fluorine, but evenperfluorinated ion-exchange polymers degrade in use. The degradation ofperfluorinated ion-exchange polymers is also believed to be a result ofthe reaction of the polymer with hydrogen peroxide.

Thus, it is desirable to develop a process for reducing or preventingdegradation of a proton exchange membrane or membrane electrode assemblydue to their interaction with hydrogen peroxide radicals, therebysustaining performance while remaining stable and viable for longerperiods of time, wherein as a result, fuel cell costs can be reduced.

SUMMARY OF THE INVENTION

The present invention relates to a membrane electrode assembly for anelectrochemical cell such as a fuel cell. The membrane electrodeassembly comprises an anode, a cathode, and an ionomer membrane disposedbetween the anode and cathode. A catalytically active component isdisposed within the membrane electrode assembly in a location selectedfrom the group of within the anode, within the cathode, within theionomer membrane, abutting the anode, abutting the cathode, abutting theionomer membrane, and combinations thereof. The catalytically activecomponent comprises particles containing: a metal oxide from the groupof alumina, titanium dioxide, zirconium oxide, germania, silica, ceria,and combinations thereof; a stabilizer from the group of metal ions andmetalloid ions, and combinations thereof; and at least one catalystdifferent from the stabilizer.

The stabilizer is preferably one or more ions containing an element fromthe group of aluminum, boron, tungsten, titanium, zirconium andvanadium. The catalyst is preferably from the group of cerium, platinum,palladium, lanthanum, yttrium, gadolinium, silver, iron, ruthenium,titanium, vanadium, and combinations thereof. It is further preferredthat the particles be colloidal particles having a mean particlediameter of less than 200 nanometers. According to a preferredembodiment of the invention, the colloidal particles contain silica andthe stabilizer includes boron ions. According to a further preferredembodiment of the invention, the colloidal particles are silicaparticles stabilized with boron ions, and the catalyst is cerium orruthenium.

The present invention also relates to a process for increasing peroxideradical resistance (i.e., increasing the oxidative stability of the ionexchange polymer in the membrane and/or electrodes of a membraneelectrode assembly) in a membrane electrode assembly having an anode, acathode, and a highly fluorinated ionomer membrane disposed between theanode and cathode, comprising the step of introducing the catalyticallyactive component described above into the membrane electrode assembly ina location selected from the group of within the anode, within thecathode, within the ionomer membrane, abutting the anode, abutting thecathode, abutting the ionomer membrane, and combinations thereof.According to one embodiment of the invention, the catalytically activecomponent is introduced into the membrane electrode assembly by imbibingthe ionomer membrane with a solution of the catalytically activecomponent in a solvent. According to another embodiment of theinvention, the catalytically active component is introduced into themembrane electrode assembly by solution casting the ionomer membranefrom a mixture of the ionomer, a solvent and the catalytically activecomponent.

Other methods, features and advantages of the present invention will beor will become apparent to one with skill in the art upon examination ofthe following detailed description. It is intended that all suchadditional methods, features and advantages be included within thisdescription and within the scope of the present invention.

DETAILED DESCRIPTION

Where a range of numerical values is recited herein, unless otherwisestated, the range is intended to include the endpoints thereof, and allintegers and fractions within the range. It is not intended that thescope of the invention be limited to the specific values recited whendefining a range. Moreover, all ranges set forth herein are intended toinclude not only the particular ranges specifically described, but alsoany combination of values therein, including the minimum and maximumvalues recited.

The present invention is intended for use in conjunction with fuel cellsutilizing proton-exchange membranes. Examples include hydrogen fuelcells, reformed-hydrogen fuel cells, direct methanol fuel cells or otherorganic feed fuel cells such as those utilizing feed fuels of ethanol,propanol, dimethyl- or diethyl ethers, formic acid, carboxylic acidsystems such as acetic acid, and the like.

As used herein, “catalytically active component” shall mean a componenthaving the ability to serve as a hydrogen peroxide scavenger to protectthe PEM from chemical reaction with hydrogen peroxide by decomposinghydrogen peroxide to 2H₂O and O₂. As noted above, and while not wishingto be bound by theory, it is believed that degradation of PEMs is aresult of the reaction of the membrane polymer with hydrogen peroxideradicals, which are generated during fuel cell operation.

Typical perfluorosulfonic acid ion-exchange membranes found in usethroughout the art will degrade over time through decomposition andsubsequent dissolution of the fluoropolymer, thereby compromisingmembrane viability and performance. However, the present inventionprovides for a membrane having a long term stability, targetingdurability goals of up to about 8000 hours in automotive fuel cellapplications and up to about 40,000 hours for stationary fuel cellapplications.

Catalytically Active Component

In general, the catalytically active components of the present inventionare delivered to the interior of the ion exchange membrane, the anodeelectrode, the cathode electrode, or the surface of a gas diffusionbacking (anode or cathode sides). The catalytically active componentsmay additionally or alternatively be provided to other locations such asto the surface of the ion exchange membrane or the electrodes. Asignificant advantage of the catalytically active components of theinvention is that the component can be incorporated into a PEM or MEAwithout the need for subsequent treatment steps such as chemicalreduction or hydrolysis treatment of a precursor to the catalyticallyactive component, which is the case with many known catalytically activecomponents.

The catalytically active component used for treating a PEM or MEAcomprise colloidal or fumed metal oxide particles such as alumina,silica, ceria (CeO₂), Ce₂O₃, titania (TiO₂), Ti₂O₃, zirconium oxide,manganese dioxide, yttrium oxide (Y₂O₃), Fe₂O₃, FeO, tin oxide,germania, copper oxide, nickel oxide, manganese oxide, tungsten oxide,and mixtures thereof. Preferred particles are colloidal particlesincluding, but are not limited to, colloidal silica, colloidal ceria,and colloidal titanium dioxide, with colloidal silica being mostpreferred. These metal oxide particles may be produced by any techniqueknown to those skilled in the art.

In preferred embodiments, the metal oxide consists of metal oxideaggregates and colloid particles having a size distribution with amaximum colloid particle size less than about 1.0 micron, and a meancolloid particle diameter less than about 0.4 micron and a forcesufficient to repel and overcome the van der Waals forces betweenparticle aggregates and/or individual particles. The particle sizedistribution in the present invention may be determined utilizing knowntechniques such as transmission electron microscopy (TEM). The meanparticle diameter refers to the average equivalent spherical diameterwhen using TEM image analysis, i.e., based on the cross-sectional areaof the particles. By “force” is meant that either the surface potentialor the hydration force of the metal oxide particles must be sufficientto repel and overcome the van der Waals attractive forces between theparticles. A spherical or approximately spherical particle is preferredin this invention.

In a preferred embodiment, the metal oxide colloid particles may consistof discrete, individual metal oxide colloid particles having meanparticle diameters from 2 nanometers to 200 nanometers, and morepreferably from 5 nanometers to 100 nanometers, and most preferably from5 to 50 nanometers.

The catalytically active component further comprises at least onestabilizer. As used herein, the term “stabilizer” means an agenteffective to help maintain the particles as a sol in an aqueous medium.Suitable stabilizers include metals and borderline metals or metaloids,from the group of boron, tungsten, aluminum, titanium, zirconium andvanadium and combinations thereof. Preferably, the stabilizer comprisesmetal ions or metalloid ions containing aluminum, boron, tungsten,titanium, zirconium, or vanadium, with boron containing ions being mostpreferred.

The catalytically active component further comprise at least onecatalyst. As used herein, the term “catalyst” means an agent effectiveto catalyze a reaction that decomposes H₂O₂. Preferred catalysts possessmultiple oxidation states, and are from the group of cerium, platinum,palladium, lanthanum, yttrium, gadolinium, silver, iron, ruthenium,titanium, vanadium, and combinations thereof. The catalysts may bepresent as metals, metal salts or metal oxides. Ruthenium and cerium arethe most preferred catalysts. The at least one stabilizer and the atleast one catalyst should not simultaneously be the same element.

In particularly preferred embodiments, the inventive compositioncomprises bimetallic surface-modified colloidal particles containing asthe two metals on the surface of the particles boron and ruthenium, orboron and cerium. It should be apparent from the foregoing that theterms “metal” and “bimetallic” as used herein in the context of surfacemodification are intended to encompass borderline metals or metalloids,such as boron, as well as more prototypical metals. Other combinationsof metals are also possible, as are combinations of metals andnon-metals.

It is preferred that at least 10%, more preferably 40-95%, even morepreferably 80-95% of available surface sites on the colloidal particlesbe occupied by the stabilizer and/or the catalyst. The percentage ofsurface sites covered on the particles in a composition of thisinvention can range up to 100%.

The molar ratio of catalyst to stabilizer can vary depending upon thecomposition of the colloidal particle. Similarly, the molar ratio ofcatalyst to colloidal metal oxide can also vary depending uponconditions and desired results. For example, the molar ratio of catalystto stabilizer preferably ranges from 1:1 to 1:10 and the molar ratio ofcatalyst to metal oxide preferably ranges from 1:1 to 1:10. In certainembodiments, the molar ratio of stabilizer to colloidal metal oxideparticle ranges from 10:1 to 1:10.

Typically, the stabilizer comprises from about 0.1 wt-% to about 20 wt-%of the catalytically active component, preferably from about 0.5 wt-% toabout 15 wt-% and more preferably from about 0.8 wt-% to about 7 wt-% ofthe catalytically active component.

Typically, the catalyst comprises from about 0.05 wt-% to about 40 wt-%of the catalytically active component, preferably from about 0.1 wt-% toabout 20 wt-% and more preferably from about 0.3 wt-% to about 10 wt-%of the catalytically active component.

The amount of surface-modification of the metal oxide particle withstabilizer depends upon the average size of the particles. Colloidalparticles that are smaller and which consequently have higher surfacearea generally require higher relative amounts of stabilizer than dolarger particles, which have lower surface area. As a non-limitingillustrative example, for boric acid surface-modified colloidal silica,the various sizes of colloidal particles require the approximate levelsof boric acid modification as shown in the table below, in order toachieve good stability towards gel formation in acidic media, such as anion-exchange polymer in proton form.

Mean Particle Relative Amount of % Modification Diameter Boric Acid toSilica if Silica (Nanometers, nm) (R, unitless) Surface* 12 8.0 92 236.0 95 50 4.3 98 100 2.0 99 R = 100 × (moles of boric acid)/(moles ofsilica) *Approximate values

The surface coverage of the surface modified metal oxide particles canbe characterized using zeta potential measurement. For example, theamount of surface coverage of boric acid on the silica surface can bemeasured using a Colloidal Dynamics instrument, manufactured byColloidal Dynamics Corporation, Warwick, R.I. The Colloidal Dynamicsinstrument measures the zeta potential (surface charge) of surfacemodified particles. During the preparation of boric acid modifiedsilica, boric acid is added to the deionized silica particles, whichchanges the zeta potential of the silica particle surface. Afterreaching full surface coverage, there is no further change in the zetapotential of the surface modified silica. From a titration curve of zetapotential as a function of grams of boric acid to a given amount ofsilica, it is possible to determine the percent surface coverage ofboric acid on the silica surface. After completing the reaction withboric acid, the surface coverage achieved by reacting the boron-modifiedsol with the second metal salt can also be determined from the zetapotential in the same way.

It is also possible to provide surface-modified metal oxide particlescontaining more than two different agents bonded to their surfaces.Thus, multi-metallic surface-modified particles containing more than twodifferent metals or metalloids on their surface are also within thescope of the invention, as are combinations of at least two differentmetals, metalloids and other organic agents such as chelating agents orcomplexing agents.

The multi-metallic surface modified particles described above can beprepared by reacting de-ionized metal oxide particles such as alumina,titanium dioxide, zirconium oxide, germania, silica, ceria and mixturesthereof in an aqueous dispersion with a stabilizer in solution such assolutions of boric acid, aluminum acetate, tungstic acid, or zirconiumacetate. In a preferred embodiment, de-ionized colloidal silicaparticles in an aqueous dispersion are reacted at about 60° C. with aboric acid solution having a pH of about 2. The stabilized particles maybe subsequently reacted with a catalyst metal salt solution or a mixtureof catalyst metal salts at ambient temperature to obtain multi-metalsurface modified particles. Examples of suitable catalyst metal saltsinclude platinum chloride, ruthenium nitrosyl nitrate and ceria acetate.The multi-metal surface modified particles can also be treated with achelating agent or complexing agent followed by reaction with additionalmetal salts.

The chemical structure of one ruthenium and boron modified silicaparticle useful in the invention is shown as structure (I) below.

The chemical structure of one ceria and boron modified silica particleuseful in the invention is shown as structure (II) below.

The catalytically active component may be homogenously ornon-homogeneously dispersed within the ion-exchange polymer of themembrane or electrodes of a membrane electrode assembly. Thecatalytically active component may be further homogeneously ornon-homogeneously dispersed, surface coated or deposited on the surfaceof the ion exchange membrane, the anode electrode, the cathodeelectrode, or the gas diffusion backing.

The amount of catalytically active component utilized is dependent uponthe method in which it is employed, for example, whether it is dispersedwithin the membrane or the electrodes, or applied onto the surface ofthe membrane, the electrodes or the gas diffusion backing.

Proton Exchange Membrane

The proton exchange membrane of the present invention is comprised of anion exchange polymer, also known as an ionomer. Following the practiceof the art, in the present invention, the term “ionomer” is used torefer to a polymeric material having a pendant group with a terminalionic group. The terminal ionic group may be an acid or a salt thereofas might be encountered in an intermediate stage of fabrication orproduction of a fuel cell. Proper operation of an electrochemical cellmay require that the ionomer be in acid form. The polymer may thus behydrolyzed and acid exchanged to the acid form.

An ionomer suitable for the practice of the invention has cationexchange groups that can transport protons across the membrane. Thecation exchange groups are acids that can be selected from the groupconsisting of sulfonic, carboxylic, boronic, phosphonic, imide, methide,sulfonimide and sulfonamide groups. Typically, the ionomer has sulfonicacid and/or carboxylic acid groups. Various known cation exchangeionomers can be used including ionomeric derivatives oftrifluoroethylene, tetrafluoroethylene, styrene-divinylbenzene, alpha,beta, beta-trifluorostyrene, etc., in which cation exchange groups havebeen introduced.

Highly fluorinated ionomers are the preferred ionomers. However, otherionomers may be utilized in the proton exchange membrane such aspartially fluorinated ionomers including ionomers based ontrifluorostyrene, ionomers using sulfonated aromatic groups in thebackbone, non-fluorinated ionomers including sulfonated styrenes graftedor copolymerized to hydrocarbon backbones, and polyaromatic hydrocarbonpolymers possessing different degrees of sulfonated aromatic rings toachieve desired range of proton conductivity in the membrane. By highlyfluorinated ion-exchange polymers, it is meant that at least 90% of thetotal number of univalent atoms in the polymer are fluorine atoms. Mosttypically, the ion exchange membrane is made from perfluorosulfonic acid(PFSA)/tetrafluoroethylene (TFE) copolymer. It is typical for polymersused in fuel cells to have sulfonate ion exchange groups. The term“sulfonate ion exchange groups” as used herein means either sulfonicacid groups or salts of sulfonic acid groups, typically alkali metal orammonium salts. For fuel cell applications where the polymer is to beused for proton exchange such as in fuel cells, the sulfonic acid formof the membrane is used. If the polymer comprising the membrane is notin sulfonic acid form when the membrane is formed, a post treatment acidexchange step can be used to convert the polymer to acid form. Suitableperfluorinated sulfonic acid polymer membranes in acid form areavailable from E.I. du Pont de Nemours and Company, Wilmington, Del.,under the trademark Nafion®.

Reinforced ion exchange polymer membranes can also be utilized in themanufacture of membranes containing the catalytically active componentsdiscussed above. Such membranes are typically reinforced with a poroussupport such as a microporous film or a woven or nonwoven fabric. Aporous support may improve mechanical properties for some applicationsand/or decrease costs. The porous support can be made from a wide rangeof materials, including hydrocarbons and polyolefins (e.g.,polyethylene, polypropylene, polybutylene, and copolymers of thesematerials) and porous ceramic substrates. Reinforced membranes can bemade by impregnating a porous, expanded polytetrafluoroethylene film(ePTFE) with ion exchange polymer. ePTFE is available under the tradename “Gore-Tex” from W. L. Gore and Associates, Inc., Elkton, Md., andunder the trade name “Tetratex” from Tetratec, Feasterville, Pa.Impregnation of ePTFE with perfluorinated sulfonic acid polymer isdisclosed in U.S. Pat. Nos. 5,547,551 and 6,110,333. The catalyticallyactive component particles can be incorporated into the ionomer beforethe porous support is impregnated with the ionomer. Alternatively, areinforced membrane can be imbibed with a solution containing thecatalytically active component.

The ion exchange membrane for use in accordance with the presentinvention can be made by extrusion or casting techniques and has athickness that can vary depending upon the intended application,typically ranging from 10 mils to less than 1 mil. The preferredmembranes used in fuel cell applications have a thickness of about 5mils (about 127 microns) or less, and more preferably about 2 mils(about 50.8 microns) or less.

Impregnation of a Membrane with at a Catalytically Active Component

The catalytically active component can be added directly to the PEM byseveral processes known in the art such as, for example, direct imbibingof a PEM or by casting or melt extruding PEMs with the catalyticallyactive component precursors incorporated in the ionomer. Typically, thecatalytically active components of the present invention comprise fromabout 1 wt-% to about 20 wt-% of the total weight of the membrane, andpreferably from about 2 wt-% to about 10 wt-% and more preferably fromabout 3 wt-% to about 8 wt-% of the membrane.

A preferred process for incorporating the catalytically active componentinto a PEM is by solution casting of the membrane. In this process, thecatalytically active component particles are mixed with the ionomer andan organic solvent or a mixture of organic solvents or water. It isadvantageous for the solvent to have a sufficiently low boiling pointthat rapid drying is possible under the process conditions employed.When flammable constituents are to be employed, the solvent can beselected to minimize process risks associated with such constituents.The solvent also must be sufficiently stable in the presence of theion-exchange polymer, which has strong acidic activity in the acid form.The solvent typically includes polar components for compatibility withthe ion-exchange polymer. A variety of alcohols are well suited for useas the organic solvent, including C1 to C8 alkyl alcohols such asmethanol, ethanol, 1-propanol, iso-propanol, n-, iso-, sec- andtert-butyl alcohols; the isomeric 5-carbon alcohols such as 1, 2- and3-pentanol, 2-methyl-1-butanol, 3-methyl, 1-butanol, etc.; the isomeric6-carbon alcohols, such as 1-, 2-, and 3-hexanol, 2-methyl-1-pentanol,3-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl, 1-pentanol,4-methyl-1-pentanol, etc.; the isomeric C7 alcohols and the isomeric C8alcohols. Cyclic alcohols are also suitable. Preferred alcohols aren-butanol and n-hexanol. The organic solvent may also include glycols tobe used alone or in combination with alcohols.

The mixture of catalytically active component, ionomer and solvent iscast onto a carrier substrate, dried to remove the solvents and thenheated at higher temperatures to coalesce the membrane. The membrane maybe solution cast in a variety of forms, including single layer films,multiple layer films, or films incorporating a reinforcing substrate orreinforcing fibers. In multiple layer films, the catalytically activecomponent particles can be selectively included in particular layers. Ina reinforced solution cast membrane, the catalytically active componentcan be incorporated on one side of the reinforcement, on both sides ofthe reinforcement or throughout the entire membrane. Alternatively,where the membrane is cast in layers, the catalytically active componentmay be selectively applied as a thin layer of catalytically activecomponent in ionomer between one or more layers of the membrane. Inaddition, different catalytically active component particles can beadded to different layers of solution cast membranes.

In order to imbibe a PEM with catalytically active component, a PEM canbe soaked in a solution of the catalytically active component in water,alcohol or a mixture thereof. The membrane is typically soaked in thesolution for 30 minutes to several hours. After soaking, the membrane isremoved from the solution and dried so as to leave the catalyticallyactive component in the membrane.

Surface Coating of Catalytically Active Components

The catalytically active components described above can be applied tothe surface of a membrane prior to the application of anelectrocatalyst; applied to the surface of the membrane as part of anelectrocatalyst layer; or applied to the surface of the electrodes orgas diffusion backing using methods known within the art for theapplication of such coatings. When the catalytically active component isapplied to the surface of the membrane, electrodes or gas diffusionbacking (GDB), the catalytically active component is mixed with ionomerand a solvent for application to the desired surface. The surface layercontaining the catalytically active component and ionomer typically hasa thickness of less than about 10 microns, and preferably from about0.01 to about 5 microns, and more preferably from about 0.5 to about 3microns.

Ink printing technology can be used for the application of the mixtureof catalytically active component, ionomer and solvent to a membrane orelectrode surface. Alternatively, a decal transfer process can be usedwherein the mixture of catalytically active component, ionomer andsolvent is applied to a release film and dried to form a decal. Theexposed surface of the decal is subsequently placed against a membraneor electrode surface and subjected to hot pressing to fix the decal tothe surface before the release film is removed. Other application andcoating techniques known within the art can also be used, such asspraying, painting, patch coating, screen printing, pad printing orflexographic printing.

Typically, a liquid medium or carrier is utilized to deliver a surfacecoating of catalytically active component and ionomer to the membrane,electrodes or GDB. Generally, the liquid medium is also compatible withthe process for creating a gas diffusion electrode (GDE) or catalystcoated membrane (CCM), or for coating the cathode and anodeelectrocatalyst onto the membrane or GDB. It is advantageous for theliquid medium to have a sufficiently low boiling point that rapid dryingis possible under the process conditions employed, provided however,that the medium does not dry so fast that the medium dries beforetransfer to the membrane or electrode surface. The medium also must besufficiently stable in the presence of the ion-exchange polymer, whichmay have strong acidic activity in the acid form. The liquid mediumtypically includes polar components for compatibility with theion-exchange polymer, and is preferably able to wet the membrane. Polarorganic liquids or mixtures thereof, such as the alcohols andalcohol/water mixtures discussed in the solution casting section above,are typically used. Water can be present in the medium if it does notinterfere with the coating process. Although some polar organic liquidscan swell the membrane when present in sufficiently large quantity, theamount of liquid used is preferably small enough that the adverseeffects from swelling during the coating process are minor orundetectable.

The catalytically active component can be applied in a number of ways tothe gas diffusion backing of a membrane electrode assembly. The gasdiffusion backing comprises a porous, conductive sheet material in theform of a carbon paper, cloth or composite structure, which canoptionally be treated to exhibit hydrophilic or hydrophobic behavior,and coated on one or both surfaces with a gas diffusion layer, typicallycomprising a layer of particles and a binder, for example,fluoropolymers such as PTFE. Where the catalytically active component isdirectly applied to the gas diffusion backing, an appropriateapplication method can be used, such as spraying, dipping or coating.The catalytically active component can also be incorporated in a “carbonink” (carbon black and electrolyte) that may be used to pretreat thesurface of the GDB that contacts the electrode surface of the membrane.The catalytically active component can also be added to the PTFEdispersion that is frequently applied to the GDB to imparthydrophobicity to the GDB.

Where the catalytically active component is applied to the surface ofthe PEM by adding it to the anode or cathode electrocatalyst electrodelayers of the membrane electrode assembly, the catalytically activecomponent comprises from about 0.5 wt-% to about 10 wt-% of the totalweight of the electrode, and more preferably from about 1 wt-% to about8 wt-% of the total weight of the electrode. Such electrode layers maybe applied directly to the ion exchange membrane, or alternatively,applied to a gas diffusion backing, thereby forming a catalyst coatedmembrane (CCM) or gas diffusion electrode (GDE), respectively. A varietyof techniques are known for CCM manufacture. Typical methods forapplying the electrode layers onto the gas diffusion backing or membraneinclude spraying, painting, patch coating and screen, decal, padprinting or flexographic printing. Such coating techniques can be usedto produce a wide variety of applied layers of essentially any thicknessranging from very thick, e.g., 30 μm or more, to very thin, e.g., 1 μmor less. The applied layer thickness is dependent upon compositionalfactors as well as the process utilized to generate the layer.

The embodiments of the present invention are further illustrated in thefollowing Examples. From the above discussion and these Examples, oneskilled in the art can ascertain the essential characteristics of thisinvention, and without departing from the spirit and scope thereof, canmake various changes and modifications of the invention to adapt it tovarious uses and conditions. Thus various modifications of the presentinvention in addition to those shown and described herein will beapparent to those skilled in the art from the foregoing description.Although the invention has been described with reference to particularmeans, materials and embodiments, it is to be understood that theinvention is not limited to the particulars disclosed, and extends toall equivalents within the scope of the claims.

EXAMPLES

The examples are directed to the preparation of metal-modified colloidalsilica and its use with perfluorinated membranes of fuel cell MEAs.Various bi-metallic surface coated colloidal silica particles wereprepared and used to treat proton exchange membranes suitable for use inMEAs. Sample membranes were tested for oxidative stability according toa hydrogen peroxide stability test.

In the hydrogen peroxide stability test, the decomposition of variousmembranes due to the action of H₂O₂ on the membrane in the presence ofFe2+ catalyst was measured. The decomposition of the membrane wasdetermined by measuring the amount of hydrogen fluoride that is releasedfrom the membrane during a reaction with hydrogen peroxide radicals.

Part A of the examples describes the preparation of bimetallic surfacecoated silica colloidal particles. Part B describes the preparation andproperties of PEMs imbibed with the bi-metallic modified silica of PartA. Part C describes the preparation and properties of solution cast PEMswith bi-metallic modified silica of Part A incorporated therein.

Part A:

Preparation of metal-boron oxide modified silica in two steps. Step 1 isdirected to the preparation of boron modified silica, and Step 2 isdirected to the immobilization of different catalyst metal ions on theboron modified silica.

Step 1: Preparation of Boron-Modified Silica. The procedure used for thepreparation of boron-coated silica was as described in U.S. Pat. No.6,743,267 directed to surface modified colloidal abrasives.Approximately 1 kg of AMBERLITE IR-120, a strongly acidic cationicexchange resin (Rohm and Haas Company, Philadelphia, Pa.), was washedwith 1 liter of 20% sulfuric acid solution. The mixture was stirred andthe resin was allowed to settle. The aqueous layer was decanted andwashed with 10 liters of deionized water. The mixture was again allowedto settle and then the aqueous layer was decanted. This procedure wasrepeated until the decanted water was colorless. This procedure affordedan acidic form of resin.

12 kg (approximately 2.27 gallons) of SYTON™ HT 50, a 50 nanometer meanparticle size colloidal silica in the sodium form (DuPont Air ProductsNanoMaterials L.L.C., Tempe, Ariz.) was placed in a five-gallon mix tankequipped with an agitator. 2.502 kg of de-ionized water was added to thetank and the solution was allowed to mix for a few minutes. The pH ofthe solution was measured to be approximately 10.2. With continued pHmonitoring, small amounts of the acid-state ion-exchange resin wereadded, while allowing the pH to stabilize in between additions.Additional resin was added in small portions until the pH had dropped topH 1.90-2.20. Once this pH limit had been reached and was stable in thisrange, no further ion-exchange resin additions were made and the mixturewas stirred for 1-1.5 hours. Subsequently, the mixture was passedthrough a 500-mesh screen to remove the ion-exchange resin and affordedde-ionized SYTON HT 50 colloidal silica.

A solution of 268 g of boric acid powder (Fisher Scientific, 2000 ParkLane, Pittsburgh, Pa.) in 5.55 kg of de-ionized water was prepared in a10 gallon mixing tank equipped with an agitator and a heater by slowlyadding the boric acid powder until all had been added to the water andthen agitating the mixture for 5 minutes and increasing the temperatureof the mixture to 55-65° C. De-ionized and diluted SYTON HT 50 (14.5 kg)was then added to the boric acid solution slowly over about 1.2 hours byadding it at approximately 200 ml/minute and maintaining the temperaturegreater than 52° C. while agitating the mixture. After this addition wascompleted, heating at 60° C. and agitation of the mixture were continuedfor 5.5 hours. The resulting solution was subsequently filtered througha 1-micron filter to afford an aqueous dispersion of boronsurface-modified colloidal silica, with about 30% solids.

This boron surface-modified colloidal silica was characterized forcolloid stability over 15 days using a Colloidal Dynamics instrument(Warwick, R.I.), and was found to exhibit both constant pH (pHapproximately 6.6) and zeta potential (zeta potential of approximately58 millivolts) over the 15-day test period. The percentage of surfacesites of this surface-modified colloidal silica occupied byboron-containing compound(s) was calculated to be approximately 98%. Themolar ratio of boric acid to silica was 4.3.

Step 2: Immobilization of Catalyst Metal Ions on the Boron-ModifiedSilica. For each example, 170 grams of de-ionized water was added to a250 ml beaker, and was kept under agitation using a magnetic stirrer. Tothe de-ionized water, 80 grams of the aqueous dispersion of boric acidmodified silica, 30% solids was added slowly, and mixed for anadditional 10 minutes. For each example, the metal salt specified inTable 1 for Examples (1-6) was added to one of the beakers underagitation to form a dispersion of bimetallic surface-modified silica.Each dispersion was agitated for an additional 15 minutes. The pH ofeach dispersion was measured, and is reported in Table 1. Table 1 alsosummarizes the amounts of the components of each dispersion.

TABLE 1 Sample Comp Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. A D.I. Water170 170 170 170 170 170 170 (grams) Metal Salt Ferric LanthanumGadolinium Platinum Ruthenium Ruthenium — nitrate nitrate nitratechloride nitrosyl- nitrosyl- nitrate nitrate Amount of 1.66 g in 0.14 gin 0.19 g in 0.16 in 2.8 g in 14.0 g in — metal salt 10% 10% 10% 10%1.5% 1.5% solution solution solution solution solution solution Boricacid 80 80 80 80 80 80 80 modified silica, 50 nm 30% solids (grams)Molar ratio 0.1 0.1 0.1 0.1 0.1 0.5 — of metal to boroncoated silica, %pH 6.35 7.7 7.2 8.0 2.0 1.5 6.3

Additional dispersions were prepared using a boron-coated colloidalsilica made by the process describe in Step 1 above except that 12nanometer mean particle size colloidal silica particles (Syton HS-40,DuPont Air Products NanoMaterials L.L.C., Tempe, Ariz.) were used inplace of the 50 nanometer colloidal silica particles. On the 12 nmcolloidal silica particles, the surface coverage of boric acid wasapproximately 46%. The molar ratio of boric acid to silica was about4.0.

To a 250 ml beaker, 110 grams of an aqueous dispersion of themetal-modified silica, 18.15 wt % solids, was added and was kept underagitation using a magnetic stirrer. For each example, to theboron-modified silica, the amount of deionized water specified in Table2 was added slowly, and mixed for an additional 10 minutes. Underagitation, the metal salt specified in Table 2 for Examples 7 and 8,respectively, was added slowly. Each dispersion was agitated foradditional 15 minutes. The pH was measured, and it is reported in Table2. Table 2 also summarizes the amounts of the components of eachdispersion.

TABLE 2 Sample Ex. 7 Ex. 8 DI Water 54.6 55.3 (grams) Metal Salt Ceriaacetate Ruthenium nitrosyl nitrate Amount of metal 35.4 g in 5% 34.7 gin 1.5% salt solution solution Boric acid 110 110 modified 12 nm silica,18.15% solids (grams) Molar ratio of 0.5 0.5 metal to boron- coatedsilica, % pH 3.6 1.15

Part B: Imbibed Membranes.

Membranes containing bimetallic-modified silica particles were preparedfor testing as follows. To a 25 mm×200 mm test tube was added a 1.0 gpiece of dried (1 hour at 90° C. in Vac oven) Nafion® N117 protonexchange membrane in the proton form and an EW of about 1050 (obtainedfrom DuPont, Wilmington, Del.) and having a thickness of about 7 mil andan area of about 28 cm². To this was added 25 mL of de-ionized water andthe amount of the bimetallic-modified silica particle dispersionindicated below for each example in order to incorporate approximately5-6 wt % of the particle additive into the Nafion® membranes. A stir barwas placed on top of the membrane to keep the membrane immersed in thesolution. The sample tube was slowly immersed in a hot water bath (85°C.) and held for 30 minutes. This process was repeated for each of theExamples 1-8 and Comparative Example A.

Ex. 1 Ferric nitrate modified boron coated silica, 0.5 g

Ex. 2 Lanthanum nitrate modified boron coated silica, 0.5 g

Ex. 3 Gadolinium nitrate modified boron coated silica, 0.5 g

Ex. 4 Platinum complex modified boron coated silica, 0.5 g

Ex. 5 Ruthenium nitrosyl nitrate modified boron coated silica (0.1%molar ratio), 0.5 g

Ex. 6 Ruthenium nitrosyl nitrate modified boron coated silica (0.5%molar ratio), 0.5 g

Ex. 7 Ceria acetate modified boron coated silica, 0.5 g

Ex. 8 Ruthenium nitrosyl nitrate modified boron coated silica, 0.5 g

Comparative Ex. A Boric acid modified silica control, 0.2 g

Each membrane was tested for oxidative stability using the followinghydrogen peroxide stability test. After removing the test tube from thehot water bath and cooling to room temperature, a solution of 25 mL of30% hydrogen peroxide and iron sulfate (FeSO4*7H₂O) (0.005 g) was addedto the test tube holding water and the membrane imbibed with thebimetallic modified silica particles. Each test tube was then slowlyimmersed in a hot water bath (85° C.) and heated for 18 hours. Eachsample was removed and when cooled, the liquid was decanted from thetest tube into a tared 400 mL beaker. The tube and membrane were rinsedwith 250 mL of de-ionized water, and the rinses were placed in the 400mL beaker. Two drops of phenolphthalein were added, and the content ofthe beaker was titrated with 0.1; N NaOH until the solution turned pink.The beaker was weighed. A mixture of 10 mL of the titrated solution and10 mL of sodium acetate buffer solution was diluted with de-ionizedwater to 25 mL in a volumetric flask. The conductivity of this bufferedsolution was measured using a fluoride ion selective electrode and theconcentration of fluoride (in ppm) was determined from the measuredconductivity using a previously generated “concentration” vs.“conductivity” calibration curve. The membrane imbibed with thebimetallic modified silica particles was allowed to air-dry and then wasoven-dried (1 hour at 90° C. in Vac oven) and weighed immediately. Apercent weight loss was calculated from the dry membrane weights.

The fluoride emission data for Examples 1-6 and Comparative Example Aare shown in Table 3 below.

TABLE 3 Fluoride Emission Membrane (mg fluoride/g) Control (no membrane)5.37 Example 1 1.20 Example 2 4.71 Example 3 4.91 Example 4 3.13 Example5 2.19 Example 6 1.30 Comparative Example A 4.81

The fluoride emission data for a second control and for Examples 7 and8, which were all tested separately under the same conditions as above,but with piece of Nafion® membrane from a different lot, are shown inTable 4 below.

TABLE 4 Fluoride Emission Membrane (mg fluoride/g) Control (no membrane)3.84 Example 7 0.78 Example 8 0.12

In the above examples, the metal modified boron coated silica particlesprotected the PEM against attack of hydrogen peroxide radicals asdemonstrated by lower emission of fluoride ions. Boron coated silicaparticles modified with iron, platinum, ruthenium and cerium wereparticularly effective in these experiments. Moreover, metal modifiedboron coated silica particles having a smaller particle size of 12 nm(Examples 7 and 8) were more effective at reducing fluoride emissionthan larger particles of 50 nm.

Part C: Solution Cast Membranes.

Solution cast perfluorosulfonic acid membranes containing differentamounts of ruthenium modified boron-coated silica particles of Example 5were prepared according to the following procedure and tested accordingto the hydrogen peroxide stability test.

To a 100 mL beaker, 50.4 grams of a 11.9 weight percent dispersion ofNafion® perfluorosulfonic acid polymer in 1-butanol was added andstirred with a magnetic stir bar. The perfluorosulfonic acid polymerresin was in the sulfonic acid form and had an 886 EW measured by FTIRanalysis of the sulfonyl fluoride form of the resin. To this mixture wasadded 0.6 grams of ruthenium modified boron coated silica particles ofExample 5, and the mixture was stirred for 30 minutes. The weight ratioof bi-metallic silica particle solids to Nafion® polymer solids in thisdispersion was 0.01. A membrane was solution cast from the dispersiononto 5 mil Mylar® A film (Tekra Corporation, New Berlin, Wis.) using astainless steel knife blade and was air-dried. The membrane wassubsequently oven-dried at 120° C. for 20 minutes, removed from theMylar® film and then annealed at 160° C. for 3 minutes. This sameprocedure for preparing solution cast membranes containing the samebimetallic-modified silica particles was repeated using the amounts inTable 5 below to prepare membranes with modified silica to Nafion®polymer weight ratios of 0.05 and 0.10.

The solution cast membranes were tested according to the hydrogenperoxide stability test procedure used in Examples 1-8, except that eachmembrane sample was tested three times using fresh hydrogen peroxide andiron (II) sulfate reagents each time. The cumulative fluoride emissionfor three testing cycles are reported in Table 5 below.

TABLE 5 Example Comp. Ex. B 9 10 11 Ruthenium modified boron- 0 0.6 3.06.0 coated silica, 10% solids (grams) Nation ® dispersion in 1- 50.450.4 50.4 50.4 butanol, 11.9% solids (grams) Weight ratio of coatedsilica 0 0.01 0.05 0.10 to Nation ® polymer solids Fluoride emission3.23 3.23 1.62 0.67 (mg fluoride/g)

Additional solution cast perfluorosulfonic acid membranes containingdifferent amounts of the ceria modified boron-coated silica particles ofExample 7 and the ruthenium modified boron-coated silica particles ofExample 8 were prepared and tested according to the procedure ofExamples 9-11 and Comparative Example B.

To a 100 mL beaker, the amount of a 11.9 weight percent dispersion ofNafion® perfluorosulfonic acid polymer in 1-butanol in Table 6 was addedand stirred with a magnetic stir bar. The perfluorosulfonic acid polymerresin was in the sulfonic acid form and had a 920 EW measured by FTIRanalysis of the sulfonyl fluoride form of the resin. To this mixture wasadded ceria modified boron-coated silica particles of Example 7(Examples 12 and 13) or the ruthenium modified boron-coated silicaparticles of Example 8 (Examples 14 and 15) and the mixture was stirredfor 30 minutes. The amounts of the metal modified boron-coated silicaparticles and the weight ratio of modified silica particle solids toNafion® polymer solids in this dispersion is reported in Table 6 below.

Membranes were cast from the dispersions onto 5 mil Mylar® A film (TekraCorporation, New Berlin, Wis.) using a stainless steel knife blade andair-dried. The membranes were oven-dried at 120° C. for 20 minutes,removed from the Mylar® film and then annealed at 160° C. for 3 minutes.This procedure for preparing solution cast membranes containingbimetallic-modified silica particles was repeated for Examples 12-15using the component amounts in Table 6 to prepare membranes.

The solution cast membranes were tested according to the hydrogenperoxide stability test procedure used in Examples 1-8, except that eachmembrane sample was tested three times using fresh hydrogen peroxide andiron (II) sulfate reagents each time. The cumulative fluoride emissionfor three testing cycles are reported in Table 6 below.

TABLE 6 Example Comp. Ex. C 12 13 14 15 Metal modified None ceria ceriaRu Ru boron-coated modified modified modified modified silica boron-boron- boron- boron- coated coated coated coated silica of silica ofsilica of silica of Ex. 7 Ex. 7 Ex. 8 Ex. 8 Metal modified 0 3.0 4.4 3.04.4 boron-coated silica, 10% solids (grams) Nafion ® 50 45.6 44.4 45.644.4 dispersion in 1-butanol, 11.9% solids (grams) Ethylene glycol 4.83.0 4.2 3.0 4.2 (grams) Weight ratio of — 0.055 0.083 0.055 0.083 coatedsilica to Nafion ® polymer solids Fluoride 6.08 0.82 0.60 0.20 0.11emission (mg fluoride/g)

What is claimed is:
 1. A proton exchange membrane for an electrochemicalcell comprising: an ionomer substrate, and a catalytically activecomponent disposed within or on a surface of the substrate, saidcatalytically active component comprising particles of a metal oxide, astabilizer comprising one or more ions containing an element from thegroup of aluminum, boron, tungsten, titanium, zirconium and vanadium,which stabilizer modifies the outer surface of the particles of metaloxide, at least one metal or metal ion catalyst different from thestabilizer and selected from the group of cerium, platinum, palladium,lanthanum, yttrium, gadolinium, silver, iron, ruthenium, titanium,vanadium, and combinations thereof, which catalyst modifies the outersurface of the particles of metal oxide; such that the catalyticallyactive components are bi-metallic or multi-metallic surface-modifiedparticles of metal oxide containing two or more than two differentmetals or metalloids on their surface, and wherein at least 10% and upto 100% of the surface sites on the particles of metal oxide areoccupied by the stabilizer or the catalyst; wherein the stabilizerincludes boron ions.
 2. The proton exchange membrane of claim 1 whereinthe catalyst is from the group of cerium, platinum, lanthanum,gadolinium, iron, ruthenium, and combinations thereof.
 3. The protonexchange membrane of claim 1 wherein the particles of metal oxide areselected from the group of alumina, silica, TiO₂, Ti₂O₃, zirconiumoxide, manganese dioxide, manganese oxide, Y₂O₃, Fe₂O₃, FeO, tin oxide,copper oxide, nickel oxide, tungsten oxide, germania, CeO₂, Ce₂O₃, andcombinations thereof.
 4. The proton exchange membrane of claim 1 whereinthe catalytically active components are colloidal particles having amean particle diameter of less than 200 nanometers.
 5. The protonexchange membrane of claim 4 wherein the colloidal particles containsilica.
 6. The proton exchange membrane of claim 1 wherein the catalystis from the group of cerium and ruthenium.
 7. The proton exchangemembrane of claim 1 wherein the catalytically active components arecolloidal particles having a mean particle diameter of less than 100nanometers, and the colloidal particles contain silica and boron ions.8. The proton exchange membrane of claim 7 wherein the catalyst is fromthe group of cerium and ruthenium.
 9. The proton exchange membrane ofclaim 7 wherein the colloidal particles have a mean particle diameter ofless than 25 nanometers.
 10. The proton exchange membrane of claim 1wherein the ionomer substrate is a hydrocarbon ionomer.
 11. The protonexchange membrane of claim 1 wherein the ionomer substrate is apartially fluorinated ionomer.
 12. The proton exchange membrane of claim1 wherein the ionomer substrate is a highly fluorinated ionomer.
 13. Theproton exchange membrane of claim 1 wherein the ionomer substrate is aperfluorinated sulfonic acid ionomer.
 14. The proton exchange membraneof claim 1 wherein the ionomer substrate is a reinforced membrane.